An invasive seastar is taking advantage of wharves in Australia to reproduce at extraordinarily high rates, fueling a population explosion of these creatures in the surrounding estuary.

The northern Pacific seastar (Asterias amurensis) invaded Australia’s Derwent Estuary a few decades ago. By 1995, their population in that area had soared to 27 million. The seastars are particularly common near wharves, raising concerns that ships could pick up these pests and spread them to other parts of the world.

A team decided to investigate how much the wharf seastars contributed to the estuary’s overall population. The researchers estimated the density of seastars and collected sample seastars at three wharf sites and three non-wharf sites. To determine each population’s “reproductive potential,” the team weighed the seastars and the organisms’ reproductive organs.

Wharf seastars had about twice the body weight and twice the reproductive organ weight of the non-wharf seastars, the study authors found. The population density at wharves was also much higher, and the wharf seastars made up an estimated 7 percent of the estuary’s population. Computer models suggested that wharf seastars produce 54 to 94 percent of the estuary population’s fertilized eggs — making “a massive and overwhelmingly large contribution to reproductive output relative to their overall abundance and the small area occupied in the Derwent Estuary,” the authors write.

The study, published in the Journal of Applied Ecology, suggests a possible solution for stemming the invasion. People could try removing seastars from wharves once a year before the organisms spawn. They could also prevent mussels, which provide an abundant food supply for seastars, from falling beneath wharves. Such solutions might be more palatable than releasing predators or pathogens to kill off the pests. — Roberta Kwok | 4 June 2012